Heating of dielectric materials



Dec. 15, 1942. A. L. M. A. ROUY HEATING oF DIELECTRIC MATERIALS Filed Nov. L 5, 1940 2 Sheets-Sheet l Dec. 15, 1942. A. 1 M. A. ROUY HEATING OF DIELECTRIC MATERIALS Filed NOV. 25, 1940 2 Sheets-Sheet 2 K'IIIIIIIIIIIIIIIIIIIIIIIIIII@ Patented Dec. 15, 1942 `UNITED STATES PATENT OFFICE HEATING F DIELEUI'RIC MATERIALS Auguste Louis Marie Antoine Rolly, New York, N. Y.

Application November 25, 1940, Serial No. 367,105

(Cl. 21B-47) 12 Claims.

This invention is directed to the heating of materials in an electrostatic iield.l More particularly the invention is directed to the heating of materials such as cork agglomerates by the heat produced therein when treated as the dielectric of a condenser having a very high frequency applied thereto.

While it has been known that dielectric materials can be heated in an electrostatic ileld. the general use of such heating has not taken place because many problems arise which'make such heating impractical. 'I'his is despite the fact that the heating of poor heat conducting bodies by the use of an electrostatic eld provides many advantages over prior known methods of heating. Such advantages are a quicker heating,

a more uniform heating, and a simplicity of the apparatus needed for the heating of the materials. l,

An object of the invention is to produce a method of heating dielectric materials in an electrostatic field such that the heating can be continued indefinitely, or can bev automatically stopped when the desired heating has been achieved. l

Another object of the invention is to eliminate power losses and burning produced by the evaporation of liquids from the materials being heated in an electrostatic ileld.

A further object of the invention is to produce a novel apparatus for heating materials under pressure in an electrostatic field.

Generally these objects of the invention are achieved byiirst providing novel circuits for the production of an alternating current through a condenser, in which the material to be heated forms the dielectric, so that the heating of material can be maintained indefintely, or can be automatically stopped when the desired heating has been achieved. The apparatus comprising the condenser is so constructai that water or other liquid evaporated from the lmaterial being heated, will not condense upon the electrode plates of the condenser and thus form drops which will cause arcing with resultant power losses and burning. Furthermore, the apparatus for heating the materials under pressure is constructed of special materials for removing the evaporated liquids without interfering with the heating.

Generally, these objects of the invention are achieved by the means described more fully with reference to the accompanying drawings, in which:

Fig. 1 is a circuit diagram of an electrical system used for automatically terminating the heating of the material when the desired heat value has been reached;

Fig. 2 is a circuit diagram illustrating the electrical system used for continuing the heating of the material for an indefinite length of time;

Fig. '3 is a diagrammatic view illustrating the Fig. 8 is a side elevational view of an electrode Y construction for preventing the formation of condensed liquids;

Fig. 9 is a cross-sectional view on the line 9 9 of Fig. 8;

Fig. 10 is a side view of an apparatus constructed according to Figs. 8 and 9;

Fig. 11 is a sectional view illustrating a mold for heating the material under pressure;

Fig. l2 is a partial plan view of a plate element used in the mold of Fig. 1l; and

Fig. 13 is a cross-sectional line along the lines |3-I3 of Fig. l2.

As illustrated in Figs. 1 and 2, the material 2 to be heated is inserted between the electrodes 4 and 6. I'he material 2 is a dielectric which with the electrodes 4 and 6 forms an electrical condenser. This condenser has applied thereto an alternating current having a frequency such that the dielectric will heat. This frequency maybe of the order of a million cycles per second, and upwards. Such heating is well known, but problems heretofore unsolved arise in the heating of certain materials, including agglomerates of cork and binders such as gelatin, pitch,

casein, natural latex, or the various synthetic latexes.

The circuit of Fig. l provides a means for automatically stopping the heating of the material 2 when all the desired heating thereof has taken place. 'I'his is done by placing the condenser in a resonant circuit, and allowing the resonance to be destroyed when all the heating desired has been obtained, because of the changed dielectric property in the heated material. In

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Fig. l the condenser formed by the dielectric and the electrodes 4 and 6 is connected in parallel with a variable inductance 8. A second inductance l is variably coupled to inductance d, the mutual inductance between these two being represented by M. Inductance l0 is connected across the output of a high frequency generator l2 which may produce a frequency ranging upwards of a million cycles per second, and may he of several thousand volts magnitude. As indicated in the drawings this generator produces a high frequency voltage independent of load impedance, such being well known in the art. Upon starting the heating process, the value of inductance 8 is adjusted so that its reactive impedance at the frequency of the supplied oscillations from generator l2, is equal to the reactive impedance of the condenser including material 2. Hence the parallel circuit comprising the condenser and inductance 8 will be resonant to the oscillations supplied from generator I2.

This circuit operates to prevent overheating of the material 2 as follows:

As the material 2 is heated, its dielectric constant changes and therefore a change is produced in the capacity of the condenser. Since the supplied frequency is constant or at least, not affected by this change, the variation in capacity on heating destroys the resonant condition between the condenser and inductance 8. As is characteristic of resonant circuits, a relatively small change in the capacity of the condenser from resonant value produces a relatively large decrease in the magnitude of the voltages appearing across the reactive impedance elements. Thus the magnitude of the high frequency voltage across the condenser will be greatly decreased due to a small change in the dielectric property of the material 2 as a result of heating.

This is of especial advantage in the heating of .g

agglomerates of cork with binders. As the heating of the material continuesthe water or other liquid in the material and the binder is evaporated and this coupled with the heating of the material nally causes the capacity of the condenser to be changed to the point where resonance between the two circuits is destroyed, thus materially lessem'ng transmission of power. Experimentally it has been determined that the desired heating of the material has been completed f In some instances it may be necessary to continue the heating despite the changes in the dielectric constant of the material 2. For this purpose the circuit of Fig. 2 has been successfully used. The material 2 and the electrodes 4 and 6 again form the condenser. This condenser is incorporated into the frequency determining circuit of a vacuum tube oscillator I4, which is composed of two 3-element vacuum tubes I6 and I8 connected in push-pull. The condenser is connected in parallel to inductance and this parallel circuit is symmetrically connected between the anodes of tubes I6 and I8. A second inductance 22 is symmetrically connected between the grids of tubes I6 and I8. Operating voltage and biasing means for the grids are provided in a conventional manner. A feed-back between the plate and grid circuits is provided in mutual inductance between the inductances 2D and 22. The frequency of the generated oscillations will be always practically identical to and dependent upon the resonant frequency of the parallel tank circuit comprising the condenser and inductance 20. As a matter of fact any conventional vacuum tube oscillator circuit can be utilized as long as the condenser may be incorporated in the plate tank circuit, and the frequency of the generated oscillations is dependent upon the value of the condenser. In the operation of Fig. 2, the material 2 will be heated by the generated oscillations impressed across the condenser. As the capacity of the condenser changes because of the heating of the dielectric material 2, the frequency of the tank circuit comprising the condenser and inductance 2B will shift. However, the frequency of the generated oscillations will also shift to this new resonant frequency of the tank circuit and thus maintain the resonant condition of the parallel circuit comprising the condenser and inductance 20. Consequently the heating of the material 2 will continue at the same rate for as long a period as desired without adjustment of the oscillator and despite any change in the dielectric properties of the material 2.

In the heating of many materials containing liquids, as for example cork agglomerates, vapors are evaporated and dissipated into the air. Very important precautions must be taken in the heating of the materials to prevent the vapors from causing electric arcs which damage the material and stop the heating by interrupting the resonance of the circuits. As the electrodes are relatively cool compared with the material being heated, the vapors which are evaporated from the material collect on the electrodes in the form of drops as shown in Figs. 3 and 4. In Fig. 3 the material 2a being heated has evaporated therefrom water vapor which collects upon the relatively cool electrode plate la in the form of drops b. This drop of moisture when of sufficient Weight as shown at b', nally becomes a freely falling drop as indicated at b". In the condenser, the electric potential between the electrode 4a and the material 2a varies as shown by the lines of equipotential A A, BB', CC', Fig. 4, and the potential gradient can attain a value of several thousand volts. Asthe drop b" is falling, it creates an arc E which will reach the material 2a and will burn or damage the same. Nearly all the power thus passes into the formation of this arc with a resultant decrease in the heating of the material and consequently power is wasted. The arc may become extinguished at a distance depending on the size of the drop, because actually the arc will vaporize a part of the water until the drop may be entirely vaporized. At that moment the arc is extinguished. Experimentally it is believed that the length of the arc is proportional to the cube of the diameter of the drop. However, power is lost and heat is diminished whether or not the drop reaches the material being heated.

Consequently, when parallel and horizontal electrodes are used, as in Fig. 4, any liquid following the vertical line FG cutting across these lines of equal potential will cause the starting of an arc, Whether the liquid is in the form of drops, streams or vapors. It is therefore desirable to construct apparatus in which the loss of power through the formation of such drops is eliminated. As shown in Figs. 5 and '7, this may be done by arranging the electrode plates at an angle to the horizontal so that the drops will not drop therefrom onto the material.

In Fig. 5 the electrode plates 25 and 28 of the condenser are placed vertically and the material through tube 4l) outside of tube 44.

2 to be heated is placed on a vertically movable table 3U, which table preferably is constructed of insulation material with zero loss at high frequency. As the drops of liquid forming on the electrodes will fall under the force of gravity along the line FG, and as this line is parallel to the line AA', BB of equal potential, it is clear that no arcs would be produced. While this apparatus is successful in general, yet other forms of apparatus are more practical for certain purposes, as it is sometimes necessary to have the electrode plates disposed horizontally rather than vertically.

One form of substantially horizontally disposed electrodes is shown in Figs. 6 and 7. In Fig. 6, one-half of the electrode is shown as consisting of a grid 32, the bars of which are composed of an electric conducting material such as copper or aluminum. As shown in Fig. '7 the two halves of the grid are connected together so that they make an angle a: with the horizontal, and condensed vapors collecting on the under surfaces of the bars will therefore trickle along the bars down to gutters 34. Thus drops of moisture will not fall toward the material 2 with the resultant electric arcs.

Another form of apparatus is illustrated in Figs. 8, 9 and 10. In this the horizontally disposed electrode plates 36 and 38 are constructed as hollow shells and are connected with a metal tube 40, formed into a coil 42 adapted to act as an inductance coil. Inside tube 40 is placed a smaller tube 44 which extendsr to and opens through a plate 46 inside each electrode 3B and 38. Consequently hot air, hot water, or other heating medium can be introduced through tube 44 and ejected against the outer face of electrodes 36 and 38, Plate 46 forms a baille by which the heating medium is caused to flow over the entire faces of the electrodes and returned The temperature of the heating medium is sufficient for evaporating any moisture which would tend to collect upon the outer faces of electrodes 36 and 38, and thus the formation of drops upon these electrodes is effectively prevented.

In Fig. 10 the installation of the tubing of Fig. 8 is further illustrated. As before stated the coil 42 forms an induction coil which may correspond, for example, with the induction coil 8 of Fig. 1, or 20 of Fig. 2. At the nodal potential point 44 of this coil, a double outlet for the tubes is formed, these outlets being connected together through an insulating coil 46 to a source 48 of hot water or hot gas. The primary inductance coil 50 is graphically illustrated and corresponds, for example, to the inductance I of Fig. 1.

In theabove apparatus it has been assumed that the material to be heated is of a self-supporting nature and does not need to be held in a form while being heated. If a material is lto be heated in a mold in order to give shape to the article produced, then additional problems arise. In the first place the mold must be composed o-f a material which will be able to absorb the moisture evaporated from the material while at the same time it must absorb Very little dielectric energy produced in the condenser, and

moreover it must be of a heat insulating nature so that there will be little heat loss by conduction from the material during the heating operation. In addition, if the material is to be heated under pressure, proper compression molds having the above requirements must be devised.

A mold adapted to be used either as an open mold or a pressuremold is illustrated in Figs. 11, 12 and 13. The material 2 to be heated is placed within the walls 60 forming the mold, and placed in the presence of the electrodes forming the condenser of which one electrode 62 is shown. If the material is to be put under pressure While being heated it is compressed by a plate 64 held in place by a bar 66. The walls forming the mold are oflspecial importance in order to meet the requirements heretofore mentioned. Any strong absorbent material having substantially zero loss at high frequency, and low heat conductivity will serve. Pumice stone, because of its hardness, workability, low heat loss, porosity, and insulating qualities, has been found by experiment to be unexpectedly useful in the formation of these molds. It has been discovered that Volvic lava rock satisfies the requirements in a substantially perfect manner. This pumice rock is so called because it is found at a place called Volvic near Riom, France. The rock is of volcanic origin, is very hard and porous, the

pores being in communication and very small. Despite the hardness of the rock it is capable of being milled and shaped into any desired form, It is quite resistant to heat and has substantially no loss when placed in a high frequency field. It is noted, however, that this rock will heat in a high frequency dioelectric eld when damp. Consequently, when the rock is used as a mold in the apparatus described, it will heat when dampened by liquid absorbed from the material being heated, but the heating of the rock will cause the dampness to be dissipated. According the heating when damp is an advantage rather than a disadvantage. It is especially noted that the dielectric properties of Volvic rock are to a great extent similar to those of cork. Consequently the rock is of especial Value when being used for a form for the dielectric heating of ,agglomerated cork and binders therefor. In practice it has been discovered that agglomerated cork blocks of large dimension cannot be successfully heated in a dielectric eld without the employment of pumice-rock as mold walls.

When materials are to be heated under compression by means of high frequency current, it is desirable to place the electrodes forming the plates of the condenser into contact with the material to be heated. Special precautions must be taken because, if, for example, an iron plate is placed in contact with cork, the cork will carbonize because of the great resistance of contact which causes a violent local heating accompanied by sparks. Therefore if the plate 64 is to .be used as an electrode as well as a pressure plate it must be constructed as shown more specifically in Figs. 12 and 13. Plate 64, as shown in these figures, is composed of an iron plate 66 having an aluminum or copper plate 68 secured to one face thereof by rivets 10. Perforations 12 extend through both plates. These perforations, of course, allow vapors to escape from the material being heated. When this plate is inserted into the mold, it is kept in place by the steel rod 66 engaging in holes in the Walls, or by any other keying means.

'I'he invention thus provides apparatus which successfully heats materials, especially such materials as agglomerated cork and binders therefor, in an electrostatic field. The length of heating of the material can be automatically cut off when the heating is finished as measured by the change in the dielectric constant of the material being heated, or the heating can be indefinitely continued. objectionable condensation of vaporized liquids is removed either by the vertical arrangement of the electrode plates, or by the use of special electrodes when the electrodes must be horizontally disposed. As cork materials of the type described are usually heated while under pressure, the objectionable vaporized liquids are removed by the employment of pumice stone or Volvic rock as the mold walls, while at the same time the pressure plates can be constructed as described so as to prevent undue local carbonization of the material. Although the resonant circuits of Figs. 1 and 2 are preferably used with the electrode structures shown in Figs. 5 to 13, inclusive, such structures are, of course, usable as electrodes in any condenser regardless of the source of electric current.

Having now described the means by which the objects of the invention are obtained, I claim:

1. In an apparatus for the heat treatment of dielectric materials, a condenser comprising a dielectric constituting the material to be heat treated, and electrodes disposed above and below said dielectric, at least the upper electrode being at an angle to the horizontal such that drops of liquid deposited upon the under surface of the upper electrode will not fall onto said dielectric.

2. In an apparatus as in claim 1, said upper electrode comprising a grid, the angle to the horizontal of which is such that said drops `will trickle along the under surface thereof to the lower edge thereof.

3. An electrode construction in the condenser of an apparatus for the heat treatment of dielectric materials comprising at least one hollow member adapted to form an electrode, and means for circulating a heating medium through said member.

4. An apparatus for the heat treatment of dielectric materials comprising a pair of hollow metal members adapted to be the electrodes of a condenser, a metal tube coiled, means placing the ends, respectively, of said coil into communication with the interior of said members, and

means for circulatingl a, heating medium through said coil and members.

5. In the apparatus of claim 4, said means comprising a second tube within said metal tube and spaced therefrom, and in communication with said members, and means forcing heating medium into said members through one of the tubes, and exhausting said medium through the other of said tubes.

6. In the apparatus of claim 4, said coil oomprising an inductance coil coupled in parallel with said members.

7. A mold for holding a dielectric material in an electrostatic field composed of a porous absorbant, rock-like substance of low heat conductivity, and substantially zero loss at hight electrical frequencies.

8. A mold for use in an electrostatic field comprising pumice stone.

9. A mold for use in an electrostatic field comprising Volvic lava rock.

10. A pressure electrode plate for a condenser in the heat treatment of dielectric materials comprising an iron plate, a sheet of metal of higher electrical conductivity thansaid iron plate secured to said iron plate, and perforatlons extending through said sheet and iron plate.

11. A method of heat treating a dielectric substance which emits volatile matter upon being heated, comprising placing electrodes on opposite sides of said substance, applying alternating current to said electrodes, and arranging said substance and electrodes with respect to each other to cause volatile matter emitted by said substance to follow the planes of equipotential between said electrodes.

12. In an apparatus for the heat treatment of dielectric materials which includes electrode means between which said materials are adapted to be disposed, and means for supplying alternating current to said electrodes, the improvement comprising means for conducting moisture emitted by said materials during heating along lines substantially parallel to the planes o! equipotential between said electrodes.

AUGUSTE LOUIS MARIE ANTOINE ROUY. 

